Isolated Switched-mode Power Supply With Output Regulation From Primary Side
Embodiments disclosed herein describe an isolated switched-mode power supply with its output regulated from the primary side, by generating a sensing current using a sensing element coupled to the output of the power supply, and measuring a scaled version of the sensing current which depends on the output voltage, and calculating an estimate voltage representing the output voltage, and regulating the output of the isolated switched-mode power supply based on the estimate voltage.
Not applicable.
FIELD OF INVENTIONThis invention relates to an isolated SMPS(switched-mode power supply) with the output regulated from the primary side.
BACKGROUND OF INVENTIONA SMPS(switched mode power supply) often has its input and output isolated to provide the required electrical safety, to be able to have different ground potentials or to minimize the possible noise coupling. An isolated SMPS usually has a power transformer with its primary side coupled to the SMPS's input and its secondary side coupled to the SMPS's output, and the power transformer delivers the power while providing the isolation from SMPS's input to output.
To regulate the SMPS's output, many prior arts used an optocoupler or a pulse transformer to pass a feedback signal from the SMPS's output to its input. The optocoupler is expensive and may wear out and change its characteristics over time to affect the control loop stability. The pulse transformer also has a shortcoming of being bulky and expensive. Some prior arts used a third winding, also called tertiary winding, in the power transformer to detect a feedback signal, but this will make it more difficult and more expensive to manufacture the power transformer.
Some prior arts did the feedback and control only at the secondary side of the power transformer, using post-regulator or post-switches, which usually resulted in additional power loss and lower efficiency. And some prior arts did output regulation from the primary side of the power transformer, by measuring a reflected output voltage coming from the secondary side. This method has some issues: first the reflected output voltage could be of high voltage value which makes the measurement difficult. Secondly the reflected output voltage did not accurately represent the actual output voltage since it also included a forward diode drop voltage at the secondary side whose variation could add substantial inaccuracy.
Hence it is highly desirable to improve techniques for an isolated SMPS to regulate its output.
This invention disclosed methods and structures for an isolated SMPS to be able to more accurately regulate its output from the primary side without using an optocoupler, additional pulse transformer or a tertiary winding, therefore to reduce cost, reduce layout size, improve reliability and improve accuracy.
SUMMARYThe methods and structures disclosed by this invention have a sensing element at the secondary side of the power transformer of an isolated SMPS(switched-mode power supply) which can generate a sensing current based on the SMPS's output voltage. A scaled version of this sensing current can be measured at the primary side of the power transformer, thus the output voltage can be estimated and hence regulated. With the SMPS's output voltage estimated and regulated from the primary side, it can also be done to estimate and regulate other SMPS's output parameters, such as output current, output power, from the primary side.
One embodiment disclosed in this invention shows an isolated single-ended flyback type SMPS with its output voltage regulated from SMPS's primary side. This embodiment has only one power switch at the primary side, which connects to the coil of the power transformer and turns on and off to make the power transformer to transfer energy.
Another embodiment disclosed in this invention shows an isolated double-ended flyback type SMPS with its output voltage regulated from SMPS's primary side. This embodiment has two power switches at the primary side to lower the voltage stress on the power switches.
One of the objectives of this invention is to provide a solution of isolated SMPS with extremely low component counts, low cost, small size, high accuracy and high reliability. This invention discloses methods and structures to get rid of the bulky and expensive optocoupler or pulse transformer which can not be easily integrated, and avoid measuring high voltage values, so more circuitry components can be integrated into a single die.
The typical application of this invention can be an AC-DC converter, an AC-DC charger, a POE(power over Ethernet) power converter, or other applications where an isolated SMPS is needed.
Embodiments of the invention relating to both structures and methods of operation may best be understood by referring to the following descriptions and accompanying drawings:
An isolated SMPS(switched-mode power supply) usually has an energy transfer element, which can be a power transformer whose primary side connected to a or a group of power switches and an input power, and a control block to control the power switch to rapidly turn on and off to transfer the energy from the primary side to the secondary side of the power transformer. The secondary side of the power transformer is usually connected to a rectifying element, such as a diode, and an energy storage element such as a capacitor. The energy will eventually be transferred to the energy storage element through the rectifying element, and can be put out as the SMPS's output. The control block can adjust the on and off time of the power switch to control how much energy to be transferred from the input power to the storage element which will affect the output voltage on the energy storage element.
In the methods and structures disclosed in this invention, there is a sensing element, at the output side of the isolated SMPS which is also the secondary side of the power transformer, coupled to the energy storage element. This sensing element can generate a sensing current based on the output voltage on the energy storage element. And there is also a measuring block, at the input side of the isolated SMPS which is also the primary side of the power transformer, coupled to the primary side of the power transformer and measure a scaled version of the sensing current to calculate an estimate voltage representing the output voltage on the storage element. The estimate voltage is used by the control block to adjust the on and off time of the power switch thus the output voltage on the storage element can be regulated against a target value.
In following paragraphs embodiments of this invention with flyback type isolated SMPS will be shown for example to explain the concept of the invention in detail. However it should be understood that it is not intended to limit the invention to the particular structures and methods disclosed, but on the contrary, the intention is to cover all the structure and method modifications, equivalents and alternatives falling within the scope of the invention defined by the appended claims.
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While the present disclosure describes several embodiments, these embodiments are to be understood as illustrative and do not limit the claim scope. The structures and methods disclosed in this invention can have many variations and modifications such as:
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- The SMPS can also use current mode control method, in addition to the voltage mode control method shown in the exemplary embodiments, especially since the current of the primary side coil of the power transformer has been measured;
- In addition to regulating the output voltage of the isolated SMPS from the primary side, the output current of the isolated SMPS can also be regulated from the primary side. This can be done because the output current is determined by the output voltage and the output power which is equal to the input power for a SMPS working in DCM mode;
- The SMPS can have many different circuit topologies, such as but not limited to: having single-ended or double-ended power switch, having high-sided or low-sided rectifying element, having synchronous rectifying element, or any combinations of them;
- The sensing element, instead of being a single resistor, can possibly be a combination of passive components such as resistor, capacitor, inductor and diode, to create a unique signature current which contains the output information, as long as the unique signature current can be measured and recognized by the measuring block at the primary side.
Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.
Claims
1. An electronic circuit, comprising:
- a switched-mode power supply with an input side and an output side isolated from each other,
- a sensing element coupled to said output side of said switched-mode power supply, generating a sensing current based on an output voltage on said output side of said switched-mode power supply, and
- a measuring means coupled to said input side of said switched-mode power supply, to measure a scaled version of said sensing current and then to calculate an estimate voltage representing said output voltage on said output side of said switched-mode power supply,
- said switched mode power supply regulating said output voltage based on said estimate voltage,
- whereby said output voltage on said output side of said switched-mode power supply can be regulated from said input side of said switched-mode power supply.
2. The electronic circuit of claim 1 wherein:
- said switched-mode power supply is of flyback type, with single-ended or double-ended power switch.
3. The electronic circuit of claim 1 wherein:
- said switched-mode power supply is of flyback type and working in discontinuous conduction mode.
4. The electronic circuit of claim 1 wherein:
- said switched-mode power supply is using voltage mode control or current mode control.
5. The electronic circuit of claim 1 wherein:
- said sensing element is a or a group of passive components.
6. The electronic circuit of claim 1 wherein:
- said sensing element includes at least a resistor.
7. The electronic circuit of claim 1 wherein:
- said measuring means includes a sample and hold means to sample said scaled version of said sensing current at a plurality of times.
8. A switched-mode power supply, comprising:
- an energy transfer element with a primary side and a secondary side isolated from each other,
- at least a power switch coupled to said primary side of said energy transfer element,
- at least a rectifying element coupled to said secondary side of said energy transfer element, and
- an energy storage element coupled to said rectifying element,
- a control means to control said power switch on and off to let energy transferred from said primary side to said secondary side of said energy transfer element, then to said energy storage element through said rectifying element, and
- a sensing element coupled to said energy storage element, generating a sensing current based on an output voltage on said energy storage element, and
- a measuring means coupled to said primary side of said energy transfer element, to measure a scaled version of said sensing current and therefore to calculate an estimate voltage representing said output voltage on said energy storage element, said estimate voltage used by said control means to control said power switch,
- whereby said output voltage on said energy storage element at said secondary side of said energy transfer element can be regulated from said primary side of said energy transfer element.
9. The switched-mode power supply of claim 8 wherein:
- said energy transfer element is transformer.
10. The switched-mode power supply of claim 8 wherein:
- said power switch is MOS transistor.
11. The switched-mode power supply of claim 8 wherein:
- said energy storage element is a capacitor or a group of capacitors.
12. The switched-mode power supply of claim 8 wherein:
- said rectifying element is diode.
13. The switched-mode power supply of claim 8 wherein:
- said sensing element is a or a group of passive components.
14. The switched-mode power supply of claim 8 wherein:
- said sensing element includes at least a resistor.
15. The switched-mode power supply of claim 8 wherein:
- said energy transfer element, said power switch, said rectifying element, and said energy storage element form a flyback type circuit.
16. The switched-mode power supply of claim 15 wherein:
- said flyback type circuit works in discontinuous conduction mode.
17. The switched-mode power supply of claim 8 wherein:
- said rectifying element is connected in a high-sided rectifying topology or a low-sided rectifying topology.
18. The switched-mode power supply of claim 8 wherein:
- said measuring means includes a sample and hold means to sample said scaled version of said sensing current at a plurality of times, to avoid the transient current glitch caused by the turning on of said power switch.
19. The switched-mode power supply of claim 8 wherein:
- said rectifying element is synchronous rectifier.
20. A method to regulate an output voltage of an output side of an isolated switched-mode power supply from an input side of said isolated switched-mode power supply, comprising steps of:
- (a) measuring a scaled version of a sensing current at said input side of said isolated switched-mode power supply,
- said sensing current generated by a sensing element coupled to said output side of said isolated switched-mode power supply, and
- said sensing current depending on said output voltage of said output side of said isolated switched-mode power supply, an input power supply voltage at
- said input side of said isolated switched-mode power supply, the resistance value of said sensing element, and a transfer ratio of said isolated switched-mode power supply, and
- (b) calculating an estimate voltage which representing said output voltage of said output side of said isolated switched-mode power supply, based on said scaled version of said sensing current measured, said input power supply, the resistance value of said sensing element, and said transfer ratio, and
- (c) regulating said output voltage of said output side from said input side of said isolated switched-mode power supply based on said estimate voltage.
International Classification: H02M 3/335 (20060101);